1. Field of the Invention
The present invention relates to an apparatus for detecting an error of a detector and, more particularly, to an apparatus for detecting not only an error of a potentiometer itself for detecting a rotating amount of a rotating body but also a defect of wirings to the potentiometer.
2. Description of the Related Art
An apparatus to which a rotating amount detector is applied is widely used in a variety of technical fields. For example, a case will be explained below wherein the rotating amount detector is applied to an X-ray fluoroscopic imaging system.
The X-ray fluoroscopic imaging system comprises a fluoroscopic X-ray television camera, an image intensifier for converting an X-ray image into an optical image, and an imaging spot shot device. In the fluoroscopic mode, the image intensifier converts an X-ray image of an object to be examined on a top plate of a bed into an optical image, and the optical image is received by the X-ray television camera to be displayed on a CRT (cathode ray tube) display apparatus, or the like.
The top plate of the bed is raisable, and a portion to be examined of an object to which a contrast medium is charged is fluoroscopically observed using the CRT display apparatus by raising the top plate of the bed. When a desired image is obtained, the spot shot device is operated to photograph the image.
FIG. 1 shows an example of the X-ray fluoroscopic imaging system. Referring to FIG. 1, reference numeral 27 denotes a support frame for supporting the entire system; 28, a bed which is raisablly supported by support frame 27; 29, a top plate which is arranged on bed 28 to be movable in a longitudinal direction (A-A+), and on which an object to be examined (patient) lies; and 29a, a step provided at one end of top plate 29. Reference numeral 30 denotes an X-ray tube, which is supported by support arm 31 provided to the side portion in the longitudinal direction of bed 28 to face top plate 29.
Bed 28 incorporates a fluoroscopic imaging system such as an image intensifier, an X-ray television camera, a spot shot device, and the like. The fluoroscopic imaging system is designed to face X-ray tube 30 in synchronism with movement of support arm 31 in directions of arrows D-D'.
In this system, a patient lies on top plate 29 while his feet are placed on step 29a. Bed 28 is then raised in a direction of arrow C to perform X-ray fluoroscopy and imaging.
Top plate 29 is movable in directions of arrows A-A' and B-B' in FIG. 1, so that a desired portion can be placed in an X-ray radiation field. The entire system including bed 28 can be raised in a direction of arrows C-C'. In a standard system, a moving amount of top plate 29 in the direction A-A' is a maximum of 1,000 mm to 1,500 mm, and the raising angle of bed 28 falls within the range of 105 degrees to 180 degrees.
In order to desirably select a portion to be diagnosed, the moving amount of top plate 29 is preferably increased as much as possible. In order to desirably position a contrast medium such as barium in a portion to be diagnosed, the raising angle range is preferably increased as much as possible. Raising angle control of the bed will be explained below. In a conventional system, the rotating shaft of a potentiometer is attached to a rotating shaft of a motor for raising the bed through a decelerator, so that the raising angle of the bed is detected by the potentiometer. If the raising angle of the bed is set at 90 degrees (standing state), the output value from the potentiometer is changed as the raising angle is increased when the raising motor is driven. When the output value has reached a value corresponding to the raising angle of 90 degrees, a raise drive control section detects this fact, and stops rotation of the motor. The raising motion of the bed is stopped at 90 degrees, and the bed is kept in position.
In the conventional system, the raising motion of the bed is controlled by a single potentiometer. Therefore, if the potentiometer is damaged by any cause or if a signal transmission line to the potentiometer is disconnected, an accident may occur. In the above-mentioned raising control of the bed, when the bed overruns without being stopped at the 90-degree position, a patient under examination may suffer from a fatal accident. In moving amount control of the top plate, such an accident may occur due to overrun of the top plate.